High temperature thermal creep of materials under non-stationary stress and/or temperature loading conditions

Abstract

The object of this paper is to describe the thermal creep behavior and the lifetime prediction of materials subjected to non-stationary tensile loading conditions. The calculations are based on HART's tensile test equation and on a phenomenological cavitation damage model. From this model the life fraction rule (LFR) is derived. Analytical expressions for the lifetimes are derived, which contain only stationary stress rupture data. The creep behavior of non-cavitating and ideally plastic materials is derived from the solution of the tensile test equation for the particular loading conditions considered. Cavitation damage is known to influence the creep behavior by reducing the load bearing capability. The corresponding constitutive equation containing the loading conditions as well as the damage function is derived.

The following loading conditions were considered: (i) creep at constant load F and temperature T; (ii) creep at linearly increasing load and T = const.; (iii) creep at constant load amplitude cycling and T = const.; (iv) creep at constant load and linearly increasing T; (v) creep at constant load and temperature cycling and (vi) creep at superimposed load and temperature cycling.